N Losses in Tile Drainage

C. Kent Keller

 

In the semi-arid Palouse region, many lower-landscape positions have been tile drained to enhance crop yield.   At Cook Agronomy Farm from 2000-2004 we monitored the discharge of a tile drain and a nearby profile of soil water for nitrate concentration ([NO3]), electrical conductivity level (EC), and water content to develop a conceptual framework describing soil nitrate occurrence and loss via subsurface pathways. Tile-drain baseflow [NO3] was consistently 4 mg-N L-1 and baseflow EC was 200 – 300 µS cm-1. Each year sudden synoptic increases in tile-drain discharge and [NO3] occurred in early winter following approximately 150 mm of fall precipitation, which saturated the soil and mobilized high-[NO3] soil water throughout the profile.  The greatest tile-drain [NO3] (20-30 mg-N L-1) occurred approximately contemporaneous with greatest tile-drain discharges. The EC decrease each year (to about 100 µS cm-1) during high discharge, a dilution effect, lagged approximately one month behind the first appearance of high [NO3] and was consistent with advective transport of low-EC water from the shallow profile under saturated conditions. Water-budget considerations and temporal [NO3] patterns suggest that these processes deliver water to the tile drain from both lower- and upper-slope positions, the latter via lateral flow during the high-flow season. Management practices that reduce the fall reservoir of soil nitrate might be effective in reducing N loading to streams and shallow groundwater in this region.

 

A tile-drained catchment at the Cook Agronomy Farm. Elevations are masl, contour interval is 2.5 m. NW-SE-running crop-rotation sections are delineated by light double lines. Drain line position is approximate.  Bulls-eye symbol indicates location of drain outlet with nearby time domain reflectometry and soil water sampling instrumentation, installed via a backhoed trench.

 

Data for the four-year study period. a) Event and cumulative daily precipitation by water year (Oct-Sept). Vertical arrows indicate time of cumulation of 150 mm of fall precipitation (pptn). Co-incident onset of high-flow season is evident in the discharge hydrograph for a 660-ha catchment (grayscale) containing the Cook Farm. b) Tile drain (TD) discharge rate and 48-h mean soil volumetric water content at selected depths; c) Tile drain nitrate concentration and nitrate-corrected electrical conductivity (EC), with solid vertical bars showing times of 150-mm precipitation accumulation from panel a.